The present invention relates generally to semiconductor assembly and packaging operations, and more particularly to apparatus and methods for tape automated bonding of semiconductor dies to lead frames and other substrates.
Improperly formed electrical connections are recognized as a dominant cause for failure of semiconductor device packages. Integrated circuit devices may include dozens, or even hundreds, of bonding pads which must be interconnected with other devices, typically by attachment to lead frames or specialized substrates. Such interconnection may be achieved by wire bonding, where small individual wires are bonded at one end to the bonding pad on the device, and at the other end to the lead frame or substrate. Alternatively, and of particular interest to the present invention, tape bonding utilizes gold or tin plated copper leads formed on a plastic tape. The leads are first bonded to the bonding pads on the semiconductor device in a single or "gang" operation. Thereafter, the semiconductor device is carried on the tape to a second location where the outer ends of the leads are bonded to a package lead frame or other substrate, also in a single operation.
Bonding of metal tape can be achieved either by thermocompression of gold-plated tape, reflow of solder-plated tape, or eutectic bonding of tin-plated tape. Of the three techniques, eutectic bonding is often preferred because it provides very reliable, low resistance contacts which are less prone to failure than contacts formed by the other two techniques. Such eutectic bonds, however, are generally more difficult to form and require much better temperature control of the bonding tool. A particular problem has arisen with certain bonding tools which are not evenly heated across the entire contact area. It has been found by the inventors herein that temperature variations on the order of .+-.10.degree. C. can result in failure to achieve eutectic bonding regardless of adjustments made to the time period of contact.
For these reasons, it would be desirable to provide bonding tools which are specifically designed for forming eutectic bonds between bonding tape leads and metal bonding pads on semiconductor devices. In particular, it would be desirable if such bonding tools were capable of maintaining a precisely controlled temperature along the entire contact area, and which further allow a rapid heat-up and cool-down to allow for short cycling time. Finally, it would be desirable if the bonding tool were easy to manufacture and allowed for very close tolerances such as those found in very large scale integrated (VLSI) circuits where the individual dies have a very large number of closely spaced bonding pads.